1. Field of the Invention
The present invention generally relates to a test probe assembly for electrical measuring and testing apparatus and, more particularly, to a test probe used to sample millimeter waves (MMW) or radio frequency (RF) signals in a microstrip integrated circuit (MIC) or millimeter wave monolithic integrated circuit (MIMIC).
2. Description of the Prior Art
The invention arose from a need to be able to make noninvasive measurements of signal voltages and frequencies of a millimeter wave hybrid or monolithic transceiver which is fabricated using microstrip technology. Such measurements are necessary to properly tune and trouble shoot the microstrip transceiver. A problem exists in microstrip waveguide circuits to provide an RF test point within the circuit which provides an accurate, calibrated and repeatable means of sampling a microstrip signal while at the same time not appreciably affecting the normal performance of the circuit. It is common, for example, in low frequency analog circuits to provide internal test points to facilitate the measurements of signal voltages in order to calibrate or trouble shoot the circuits. However, at microwave and millimeter wave frequencies, it is not possible to merely insert test points in the microwave circuit without greatly affecting the performance of the circuit and distorting the very signals which are desired to be measured.
A few examples of the prior art will illustrate the point. U.S. Pat. No. 4,070,619 to Trefney discloses a voltage standing wave ratio (VSWR) meter which uses an inductive loop in a microstrip configuration to provide the signal input to the meter. While the internal circuitry uses microstrip construction, the VSWR meter is for use in coaxial measurement systems, not microstrip circuits.
U.S. Pat. No. 4,211,911 to Dehn discloses a microwave directional coupler which comprises a microstrip transmission line terminating in a pair of detectors and mounted over apertures in a waveguide. The approach taken by Dehn is applicable for use in waveguide systems only, and the radio frequency (RF) sensing mechanism is not transferrable to microstrip circuits.
U.S. Pat. No. 3,704,410 to O'Brien and U.S. Pat. No. 3,753,086 to Shoemaker, Jr., show examples of microwave detector circuits using conventional waveguide structures. O'Brien uses a bridge technique which is designed to measure characteristics of passive elements; e.g., attenuation. It is unsuitable for use in measuring parameters such as power and frequency of active transceivers while operating. Shoemaker, Jr., discloses an apparatus which is limited in use in waveguide systems only; microstrip millimeter wave circuits are a unique problem not addressed by this patent.
U.S. Pat. No. 4,438,395 to Ekdahl discloses a capacitively-coupled inductive sensor for a coaxial line. The Ekdahl approach is limited to operating in transmission lines with outer conductors, which is not applicable to microstrip circuits.
U.S. Pat. No. 4,535,307 to Tsukii discloses a microwave package which is useful for testing and calibrating microwave circuits. A device to be tested is placed within a test fixture, thereby making tests of the device while in operating service impossible.